This invention generally relates to method and systems for fabricating holographic elements; and more specifically, to a method and system for fabricating a copy of a multiple holographic element.
Holographic optical elements are fabricated by the recording of a coherent laser reference beam with a coherent laser signal beam to form a hologram in a photographic recording material. More specifically, in the construction of holographic optical elements, a pair of collimated construction beams, referred to as the signal beam and the reference beam, respectively, are projected so that they overlap, at a relative angle to each other, on a recording medium, producing an optical interference pattern that is recorded in the medium as an amplitude and/or phase distribution of closely spaced lines. The signal beam may be spatially modulated by passing it through an image of a selected object; and the signal and reference beams then combine at the recording medium to produce a diffraction pattern, or hologram, unique to the selected object. Preferably, the construction beams are from the same source of coherent electromagnetic radiation, which, for example, nay be a laser. The recording medium, as is well-known, can be a photographic emulsion, dichromated gelatin, a photopolymer and the like, and can be coated or mounted on a suitable substrate such as a glass plate or a thin film.
A hologram may be used, or played back, by directing a collimated beam, referred to as the playback beam, through the recorded hologram. The hologram diffracts the playback beam and produces, at a given distance and angle from the hologram, an image of the image used to spatially modulate the signal beam employed to fabricate the hologram.
For many applications, it is beneficial to form multiple holograms on one recording medium; and typically, this is done by one of two procedures, referred to as the step and repeat process, and the parallel or coherent process, respectively. In the former process, the holograms are formed one at a time in the recording medium. This may be done, for example, by directing the signal beam through an image and then onto the recording medium to form a first hologram in the recording medium, and repeating this step a multitude of times, each time with the signal beam being directed to the same recording medium. In the parallel process of forming a multiple holographic element, the multiple holograms are all formed simultaneously. This may be done by separating the signal beam into a matrix of component beams, and directing a single image onto the recording medium to form the multiple holograms therein.
As a general rule, multiple holographic elements made by the step and repeat process have a high accuracy but a low efficiency, while multiple holographic elements made by the parallel process generally have a high efficiency but a low accuracy. To elaborate, the efficiency of a multiple holographic element is measured as the ratio of the power of the input playback beam to the power of the combined first order output beams of the hologram, and the accuracy of a hologram is measured as the ability of the element to reproduce accurately the image used to form the hologram.
In a multiple holographic element made by the step and repeat process, each hologram, because it is made separately, shares the same medium as the other holograms on the element and is able to reproduce with a high degree of accuracy the image used to form the hologram. However, because each hologram is allotted only a part of the dynamic range of the recording medium, the efficiency of the hologram is low. In a multiple holographic element made by the parallel process, because each hologram shares in the overall dynamic range of the recording medium, the hologram has a comparatively high efficiency. However, the holograms also overlap in their sharing of the dynamic range of the recording medium, and each hologram may appreciably distort its neighboring hologram. When a given hologram is played back, the distortions in the hologram caused by sharing the dynamic range of all the holograms on the recording, may appreciably reduce the ability of the given hologram to reproduce accurately the image used to form the given hologram.